An optical semiconductor relay is generally provided with an input light emitting element such as an LED (hereinafter, referred to as LED), an output photodiode array (hereinafter, referred to as PDA) and a switching MOSFET. The PDA receives input light from the LED and converts the light into a voltage. This voltage becomes a gate voltage of the switching MOSFET, and the optical semiconductor relay performs the switching by driving the MOSFET. Since electrical signals are converted into light to be transmitted, it is possible to insulate between the input and output electrically. At the same time, it is possible to drive a small-size optical semiconductor relay with low power consumption at high speed without contact wear, unlike a mechanical relay.
An optical semiconductor relay is disclosed in U.S. Pat. No. 5,013,926 (FIG. 4). FIG. 1 shows the circuit structure of such an optical semiconductor relay. As shown in the FIG. 1, an anode end of a first PDA 2 which receives light from an LED 1, a diode 5 and a gate terminal of a MOSFET 4 are connected in series. A cathode end of the first PDA 2 is connected to the source terminal of a MOSFET 4. The anode end of first PDA 2 is connected to a cathode end of a second PDA 3, and an anode end of second PDA 3 is connected to a cathode electrode of diode 5 through an impedance element 10. First PDA 2 and second PDA 3 have a plurality of photodiodes respectively the same polarities of which are connected in series. Connection area of the photodiodes constituting the second PDA 3 is smaller than those of first PDA 2. First PDA 2 and second PDA 3 are connected in series as a whole.
A normally-on transistor (hereinafter, referred to as J-FET 12) constituting a control circuit with impedance element 10 is connected as to enclose impedance element 10 by the gate and source terminals thereof. The control circuit controls speed and photosensitivity of the optical semiconductor relay by controlling charging and discharging time of the switching MOSFET constituting the optical semiconductor relay.
In this optical semiconductor relay, when an input electrical signal is turned on (ON-state), LED 1 emits light, and first PDA 2 received this light converts the light into an electrical signal. Current flows to the gate terminal of MOSFET 4 through diode 5, and the gate of MOSFET 4 is charged and shifted to an ON-state. Meanwhile, part of current converted at first PDA 2 is supplied to the gate terminal of J-FET 12 through impedance element 10. Second PDA 3 also receives the light from LED 1 and generates a current. This current is supplied to the gate terminal of J-FET 12 with the current converted at first PDA 2. A potential difference between both ends of impedance element 10 is applied between the gate and source of J-FET 12 as a bias voltage, J-FET 12 is shifted to an OFF-state.
When the input electrical signal is turned off (OFF-state), the optical signal is not output from LED 1, and the electrical signals from first PDA 2 and second PDA 3 is not output either. Accordingly, the current vanishes, and the bias voltage of J-FET 12 becomes 0 volt. Thus, J-FET 12 is shifted to an ON-state. When J-FET 12 shifts to an ON-state, it is short-circuited between the gate and the source of MOSFET 4. Therefore, a stored electric load is quickly discharged, and MOSFET 4 is shifted to an OFF-state.